This invention relates to a non-aqueous electrolyte secondary cell and, more particularly, to a non-aqueous electrolyte secondary cell employing a carbonaceous material for its negative electrode.
Recently, a proliferation of video camera and audio cassettes has caused an increased demand for reusable, rechargeable secondary cells to take the place of primary cells which are disposable.
Most of the secondary cells, now in use, are nickel-cadmium cells employing an alkaline electrolyte solution. These secondary cells have a voltage of approximately 1.2 V and it is difficult to raise their energy density. In addition, these secondary cells have a high self-discharge rate at ambient temperature of not less than 20% per month.
For this reason, non-aqueous electrolyte secondary cells employing a non-aqueous solution as an electrolyte and light metal such as lithium for a negative electrode are currently being investigated. These non-aqueous electrolyte secondary cells have a high voltage of 3 V and hence a high energy density. They also provide advantages such as low self-discharge rate and lightness of weight. However, if the non-aqueous electrolyte secondary cell employing lithium for its negative electrode is charged and discharged repeatedly, metal lithium tends to undergo dendritic crystal growth from the negative electrode until it contacts with the positive electrode. The dendritic crystal growth causes shorting in the inside of the cell which leads to difficulties in practical utilization of the cell.
To overcome this problem, non-aqueous electrolyte secondary cells in which lithium is alloyed with other metals and the resulting alloy is used for the negative electrode are also being investigated. However, these cells have a defect in that, if the cell is charged and discharged repeated, the alloy constituting the negative electrode tends to be comminuted in size, again leading to difficulties in practical utilization of the cell.
Also proposed are non-aqueous electrolyte secondary cells employing a carbonaceous material, such as coke, as an active negative electrode material. These non-aqueous electrolyte secondary cells provide for doping/undoping of lithium ions to and from the spacing between carbon layers. Accordingly, these cells are no susceptible to precipitation of metal lithium nor to alloy comminution as occurs with cells employing metal lithium or lithium alloy as the active negative electrode material. These cells exhibit optimum cyclic characteristics. If a lithium/transition metal composite oxide represented by Li.sub.x Mo.sub.2 where M denotes one or more transition metals and x is such that 0.05.ltoreq.x.ltoreq.1.10 is used as an active positive electrode material, as disclosed in JP Patent Kokai Publication JP-A-63-135099 (1988) or JP Patent Kokai Publication JP-A-1-304664 (1989), the cell capacity may be improved, so that it becomes possible to produce a non-aqueous electrolyte secondary cell having high energy density.
However, as compared to the non-aqueous electrolyte secondary cell employing metal lithium or lithium alloy as the active negative electrode material, the non-aqueous electrolyte secondary cells, employing a carbonaceous material as the active negative electrode material, are inferior in energy density, although they are superior in cyclic service life and safety.
One of the reasons for this is carbonaceous materials typically employed as the active negative electrode material comprise slurries of powders of the carbonaceous material kneaded with a binder or a dispersant. The slurries are coated on a current collector or are directly molded to form a negative electrode. The concentration of active material present in the slurries is decreased by an amount corresponding to the binder content. The binder which may comprise 10 to 20% of the negative electrode does not contribute to the cell capacity.
Efforts to improve the energy density of these cells have included attempts to increase the packing density of the carbonaceous materials. However, packing density cannot be raised beyond a certain limit value, which impedes any further increase in the energy density.